1. Field of the Invention
This invention relates in general to electrophotographic imaging members. More specifically, the invention relates to an electrophotographic imaging member having improved resistance to light shock and a method of using the imaging member.
2. Discussion of Related Art
In the art of electrophotography, an electrophotographic plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging the imaging surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated area. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable electrophotographic imaging members.
The electrophotographic imaging members may be in the form of plates, drums or flexible belts. These electrophotographic members are usually multilayered photoreceptors that comprise a substrate, a conductive layer, an optional hole blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, an optional overcoating layer and, in some belt embodiments, an anticurl backing layer.
Photoreceptors are susceptible to varying degrees of light shock, depending on the type of charge generating layer used in the photoreceptor. Light shock is a phenomenon in which a photoreceptor exposed to room light exhibits an increase in dark decay and depletion when subsequently utilized in an electrophotographic imaging process conducted by an electrophotographic imaging device such as a printer, copier or duplicator. Such exposure to light may occur, for example, during installation of the photoreceptor or during servicing of the device. It is believed that light shock can be defined/quantified in terms of exposure time, dark rest time, and V.sub.ddp voltage differential (exposed area versus unexposed area).
Due to light shock, areas of the photoreceptor that are rendered electrically conductive by exposure to room light remain conductive after termination of the exposure event. For photoreceptors susceptible to light shock, particularly for very large photoreceptor belts such as a 10 pitch belts, exposure to light results in different degrees of light exposure for different regions of the photoreceptor, e.g., the top, sides and bottom of the photoreceptor belt experience different degrees of light shock. Thus, for example during belt replacement or machine maintenance, non-uniform exposure of the photoreceptor to room light leads to non-uniformity in V.sub.ddp (dark development potential).
V.sub.ddp refers to the potential attained at the development station without the photoreceptor being exposed to light. Typical values of V.sub.ddp may be between about 600 and about 1000 volts in a given machine. V.sub.ddp registers two types of changes with cycling. In the first change, after initial exposure, the dark decay undergoes changes in a few cycles and thereafter becomes stable at a crest value. The second is a long term effect which manifests itself as a gradual decrease in V.sub.ddp (increase in dark decay) over many tens of kilocycles.
A 5 V.sub.ddp voltage differential between exposed areas and unexposed areas of a photoreceptor is undesirable because it leads to non-uniform image potentials which, in turn, leads to the formation of non-uniform toner images when the light shocked photoreceptor is subsequently utilized for electrophotographic imaging.
The light shock problem is particularly serious in photoreceptors containing hydroxygallium phthalocyanine or alkoxygallium phthalocyanine particles as photogenerating pigments, for example dispersed in a polymer binder in the charge generating layer. For very high quality imaging, this non-uniformity is extremely undesirable.
The dramatic variation in conductivity due to light shock cannot be compensated with automatic controls even in highly complex and sophisticated machines. It is therefore desired to develop a photoreceptor resistant to light shock.
U.S. Pat. No. 5,164,276 describes charge generating layers and charge transport layers for electrophotographic imaging members in which the charge generation layer or charge transport layer includes a dopant of organic molecules containing basic electron donor or proton acceptor groups. Preferred dopants include aliphatic and aromatic amines, more preferably, triethanolamine, n-dodecylamine, n-hexadecylamine, tetramethyl guanidine, 3-aminopropyltriethoxy silane, 3-aminopropyltrihydroxysilane and its oligomers. Doping of the charge generating layer is preferred (column 4, line 67 to column 5, line 3). The dopants are not identified to provide light shock resistance, and hydroxygallium phthalocyanine is not identified as a photogenerating particle to be included in the charge generating layer.
U.S. Pat. No. 5,521,306 describes a process for preparation of Type V hydroxygallium phthalocyanine comprising the in situ formation of an alkoxy-bridged gallium phthalocyanine dimer, hydrolyzing the dimer to hydroxygallium phthalocyanine and subsequently converting the hydroxygallium phthalocyanine product obtained to Type V hydroxygallium phthalocyanine.
U.S. Pat. No. 5,492,785 describes an electrophotographic imaging member having an imaging surface adapted to accept a negative electrical charge, the electrophotographic imaging member comprising a metal ground plane layer comprising at least 50 percent by weight zicronium, a siloxane hole blocking layer, an adhesive layer comprising a polyacrylate film forming resin, a charge generation layer comprising benzimidazole perylene particles dispersed in a film forming resin binder of poly(4,4'-diphenyl-1,1'-cyclohexane carbonate), and a hole transport layer, the hole transport layer being substantially non-absorbing in the spectral region at which the charge generation layer generates and injects photogenerated holes but being capable of supporting the injection of photogenerated holes from the charge generation layer and transporting the holes through the charge transport layer.
U.S. Pat. No. 4,599,286 describes an electrophotographic imaging member comprising a charge generation layer an a charge transport layer, the transport layer comprising an aromatic amine charge transport molecule in a continuous polymeric binder phase and a chemical stabilizer selected from the group consisting of certain nitrone, isobenzofuran, hydroxyaromatic compounds and mixtures thereof. An electrophotographic imaging process using this member is also described.
U.S. Pat. No. 4,265,990 describes a photosensitive member having at least two electrically operative layers. The first layer comprises a photoconductive layer and the second layer comprises a charge transport layer. The charge transport layer comprises a polycarbonate resin and a diamine having a certain specified structure. Also, metal phthalocyanines are disclosed as useful as charge generators. A photoconductor particle size of about 0.01 to 5.0 micrometers is mentioned.
As described above, there is a continuing need for versatile high quality photoreceptors that are resistant to light shock.